Coaxial cables are used for wiring inside or between electronic devices and for transmission of high-speed signals. Generally, such coaxial cables have a structure in which a central conductor is covered with an insulator, an outer circumference of the insulator is covered with an outer conductor, and an outer side thereof is covered with a protective jacket, and an outer diameter of the cable varies from 0.25 mm to several millimeters depending on use. For such coaxial cables to obtain good electrical properties with a small diameter, it is considered important that the insulator with which the outer circumference of the central conductor is covered have as low permittivity as possible.
Conventionally, resins having low permittivity, such as a fluororesin and a polyolefin resin, are used as the insulator of coaxial cables. To further lower its permittivity, in some cases, the insulator to be used is foamed by gas foaming, chemical foaming or the like. However, it is difficult to stabilize a shape during an insulator coating by a foaming extrusion, which is likely to result in a fluctuation in an outer diameter of the insulator. Further, as an extent of foaming is increased, a foamed condition becomes likely to be deteriorated, which degrades stability of longitudinal transmission characteristics and the like. Furthermore, the adhesion strength of a foamed insulator to a conductor is low.
On the other hand, as shown in (A) of FIG. 2, there is known a coaxial cable having a structure in which a plurality of hollow portions are formed along a longitudinal direction of an insulator (see, e.g., Patent document 1). In this coaxial cable 1a, the insulator 3 for a central conductor 2 has a configuration in which an inside annular body 3a adhered to the central conductor 2 and an outside annular body 3b, on which an outer conductor 5 is wound, are connected to each other via a plurality of ribs 3c such that the plurality of hollow portions 4, each having a fan-shaped cross section, are provided. The hollow portions 4 occupy 40% or more of the insulator 3. An outer circumference of the outer conductor 5 is covered with a protective jacket 6, whereby the entire cable is protected.
Further, as shown in (B) of FIG. 2, there is known a differential transmission cable 1b having a structure in which an insulator 7, which insulates a central conductor 2a, is formed with a plurality of void portions 8 along a longitudinal direction (see, e.g., Patent document 2). In this differential transmission cable 1b, the insulator 7 surrounding the central conductor 2a has a configuration in which six void portions 8, each having an elliptical cross section, are evenly arranged around the central conductor 2a. A pair of signal lines, each having the central conductor 2a insulated with the insulator 7, is shielded by an outer conductor 5a together with a drain wire 9, and the outer circumference thereof is covered with a protective jacket 6a. 